FIG. 1 illustrates a simplified circuit diagram of a current mode (up-converting) passive mixer 100. Input signals 112, 114, 116, 118 are mixed with local oscillator waveform signals 122, 124, 126, 128, by switching devices 132, 134, 136, 138 to generate an (up-)converted signal 130 that is output through an LC network 140. The LC network illustrated in FIG. 1 comprises a resonant LC L-network consisting of a series inductor 142 and a shunt capacitor 144, that presents a low impedance relative to the baseband impedance. In particular, the LC L-network consisting of the series inductor 142 and shunt capacitor 144 can provide both a low input impedance and develop a voltage at its output (current to voltage conversion). The LC network illustrated in FIG. 1 further comprises a series capacitor 146 and shunt inductor 148 that form a high pass filter that is desirable in filtering unwanted frequency mixing products, for example those close to the input signals 112, 114, 116, 118. Furthermore, the shunt inductor 148 can be used to resonate with a capacitive input of a load circuit (not shown), such as a radio frequency (RF) amplifier.
It is well known that the performance of a passive mixer, and in particular the linearity and noise performance, is highly dependent on the local oscillator waveform signals 122, 124, 126, 128. In particular, the linearity of the passive mixer 100 of FIG. 1 is optimum when the local oscillator waveform signals 122, 124, 126, 128 have fast rise and fall times, and the voltage swing VL to VH for the local oscillator waveform signals 122, 124, 126, 128 is maximized.
Fast rise and fall times are a function of the process technology and device size, and modern CMOS (complementary metal oxide semiconductor) processes are capable of producing very fast rise and fall times. However achieving a large (maximized) voltage swing can be problematic. Using standard CMOS circuits, the voltage swing range (VL to VH) is typically set by the supply voltages (VDD, VSS) of the local oscillator circuits. For power consumption reasons, it is desirable to keep the supply voltage as low as practically possible, for example as determined by the local oscillator noise requirements. However, there is a clear conflict between such low power consumption considerations and the need to maximize the voltage swing of the local oscillator waveform signals 122, 124, 126, 128.
One known solution to this conflict is to introduce a voltage level shift between the last local oscillator circuit stage and the driver circuit for the mixer devices to increase the voltage swing of the local oscillator waveform signals 122, 124, 126, 128. FIG. 2 illustrates a first conventional local oscillator waveform signal voltage shift circuit 200. A first problem with the use of such a voltage shift circuit 200 is that it requires differential input signals 210, 220, which have to be generated in the preceding stage (not shown) of the local oscillator circuit, increasing power consumption and adding complexity to the local oscillator circuit. A second problem with the use of such a voltage shift circuit 200 is that a second, higher voltage supply VDDHIGH is required, which can complicate the overall system design.
FIG. 3 illustrates an alternative conventional voltage shift circuit 300. Advantageously, the voltage shift circuit 300 illustrated in FIG. 3 is able to operate with a single ended input signal 310 and relies on AC (alternating current) coupling and a self-biased CMOS inverter. However, a problem with the voltage shift circuit 300 illustrated in FIG. 3 is that it can suffer from poor noise performance, depending on the input voltage swing and thus on how hard the gates of the transistors 320, 330 are driven. A further problem with the voltage shift circuit 300 illustrated in FIG. 3 is that it also requires a second, higher voltage supply VDDHIGH.